The invention relates to the field of a scanning system for detecting defects in a chargeable surface. In particular, this disclosure relates to a contactless system and method for sensing electrostatic surface potentials with very high spatial resolution.
In the art of xerography, a xerographic plate or photoreceptor having a photoconductive insulating layer is provided. An image is acquired by first uniformly depositing an electrostatic charge on the imaging surface of the xerographic plate and then exposing the plate to a pattern of activating electromagnetic radiation, such as light, which selectively dissipates the charge in the illuminated areas of the plate while leaving behind an electrostatic latent image in the non-illuminated areas. This electrostatic latent image may then be developed to form a visible image by depositing finely divided electroscopic marking particles on the imaging surface. It is important in research and development, engineering and quality control to be able to detect the latent image with a high spatial resolution prior to development with toner. Additionally, high spatial resolution also increases the accuracy of results obtained when studying lateral charge migration (LCM), a subset of the stability, accuracy and uniformity of the charge distributions that make up the latent image.
A photoreceptor layer for use in latent image detection in xerography or in latent image studies may be a homogeneous layer of a single material such as vitreous selenium, or it may be a composite layer containing a photoconductor and another material. One type of composite photoconductive layer commonly used in photoreceptors is described in U.S. Pat. No. 4,265,990, the entire disclosure thereof being incorporated herein by reference. The patent describes a photosensitive member having at least two electrically operative layers. One layer comprises a photoconductive layer which is capable of photo-generating holes and injecting the photogenerated holes into a contiguous charge transport layer. Generally, where the two electrically operative layers are positioned on an electrically conductive layer with the photoconductive layer sandwiched between a contiguous charge transport layer and the conductive layer, the outer surface of the charge transport layer is normally charged with a uniform electrostatic charge, and the conductive layer is utilized as an electrode. In flexible electrophotographic imaging members, the electrode is normally a thin conductive coating supported on a thermoplastic resin web.
The conductive layer may also function as an electrode when the charge transport layer is sandwiched between the conductive layer and a photoconductive layer which is capable of photogenerating electrons and injecting the photogenerated electrons into the charge transport layer. The charge transport layer in this embodiment must be capable of supporting the injection of photogenerated electrons from the photoconductive layer and transporting the electrons through the charge transport layer. The photoreceptors are usually multilayered and comprise a substrate, an optional conductive layer (if the substrate is not itself conductive), an optional hole blocking layer, an optional adhesive layer, a charge generating layer, and a charge transport layer and, in some belt embodiments, an anti-curl backing layer.
One of the first techniques for electrostatic sensing was the stylus scanner. This technology had high resolution, as limited by the stylus diameter, but was not suitable for scanning larger areas due to its overall low speed.
Next came the charge deficient spot (CDS) scanner which was developed to address the low speed limitation of the previous stylus scanners. However with the CDS scanner, it was noted that variations in the air gap distance introduced some measurement errors.
The implementation of aerodynamic floating, or the floating probe scanner (FPS) enabled the ability to better control the air gap distance, improving the overall accuracy of the measurement.
From here, two different development branches for this technology began to take shape. The first branch consisted of FPS's with look-up-table corrections for small non-uniformities in gap distance which could be used to study charge deficient spots (CDS's), which are point-like electrical defects in the photoreceptor surface or bulk. This measurement requires knowledge of the air gap for each measurement point and is determined through some signal-processing operations and a pre-determined calibration curve. These innovations were disclosed in U.S. Pat. No. 7,271,593 which is incorporated herein by reference in its entirety. The second branch has led to high resolution electrostatic (floating probe) scanners (Hi-RES) and the current invention described in detail below.
Another technique for sensing electrostatic surface potentials has been the use of current generation non-contact electrostatic voltmeters (ESV). Such devices such as those made by Trek, Inc. such as the Trek 344 with a Trek 6000B-8 probe, or the Trek 368A with a Trek 3800E-2 probe, are well known to those in the xerography community and are relatively inexpensive, can scan large amounts of area, and readily suitable for general xerography. However, ESV devices have spatial resolution that is only on the order of millimeters, which is unsuitable for LCM studies.
Another technique for sensing electrostatic surface potentials has been the use of atomic force microscopy (AFM). AFM devices provide very high spatial resolutions of up to 1 μm, however they are very expensive to install and use and can only scan small amounts of area at a time which makes them highly undesirable for applications such as latent image detection in xerography or latent image studies.
However in the context of product development or quality control, machine sensing of electrostatic surface potentials is a laborious and time consuming process involving hand feeding of sheets by test personnel along with constant monitoring of the final quality of every sheet. The prior art technology is unsuitable for product development or quality control applications because the charge amplifier sub-system is only sensitive to transient events, such as when a point-like defect is detected. The corrected floating probe scanners cannot resolve the electrostatic potential when uniform over large areas due to the AC-coupled nature of the charge amplifier. Moreover, accuracy of the test results depends a great deal upon interpretations and behavior of the personnel that are feeding and evaluating the sheets.
Further, since machine characteristics vary from machine to machine for any given model or type, reliability of the final test results for any given machine model must factor in peculiar quirks of that specific machine versus the characteristics of other machines of the same model or type. Because of machine complexity and variations from machine to machine, the data from a test in a single machine is not sufficiently credible to justify the scrapping of an entire production batch of photoreceptor material.
Thus, tests are normally conducted in three or more machines. Since a given photoreceptor may be used in different kinds of machines such as copiers, duplicator and printers under markedly different operating conditions, the sensing of electrostatic surface potentials based on the machine tests of a representative test photoreceptor sample is specific to the actual machine in which photoreceptors from the tested batch will eventually be utilized. Thus, photoreceptor tests on one machine do not necessarily predict whether the same electrostatic surface potential will occur if the same type of photoreceptor were used in a different type of machine.
Thus, for a machine latent image test, the test would have to be conducted on each different type of machine. This becomes extremely expensive and time consuming. Moreover, because of the length of time required for machine testing, the inventory of stockpiled photoreceptors waiting approval based on life testing of machines can reach unacceptably high levels. For example, a batch may consist of many rolls, with each roll yielding thousands of belts.
One test method utilizes a stylus scanner such as that described by Z. D. Popovic et al., “Characterization of Microscopic Electrical Defects in Xerographic Photoreceptors”, Journal of Imaging Technology, vol. 17, No. 2, April/May, 1991, pp. 71-75. The stylus scanner applies a bias voltage to a shielded probe, which is immersed in silicone oil and is in contact with the photoreceptor surface. The silicone oil prevents electrical arcing and breakdown. Current flowing through the probe contains information about defects, and scanning speeds up to 6×6 mm2 in about 15 minutes were achieved. Although the stylus scanner is a highly reproducible tool which enabled some important discoveries, it has the basic shortcoming of low speed.
Many attempts have also been made in the past to reduce the time of scan by designing contactless probes. For example, a probe has been described in the literature and used for readout of xeroradiographic (X-ray) amorphous selenium plates, (see, e.g., W. Hillen, St. Rupp, U. Schieble, T. Zaengel, Proc. SPIE, Vol. 1090, Medical Imaging III, Image Fonnation, 296 (1989); W. Hillen, U. Schieble, T. Zaengel, Proc. SPIE, Vol. 914, Medical Imaging II, 253 (1988); and U. Schieble, T. Zaemge, Proc. SPIE, Vol. 626, Medicine XIV/PACS IV, 176 (1986)). These probes rely on reducing the distance of a probe to a photoreceptor surface in order to increase resolution of the measurements. The typical distance of the probe to the photoreceptor surface is 50-150 micrometers. In order to avoid air breakdown, the ground plane of a xeroradiographic plate is biased appropriately to provide approximately zero voltage difference between the probe and photoreceptor surface.
In U.S. Pat. Nos. 6,008,653 and 6,119,536, the contents of both of which are incorporated herein by reference in their entirety, a contactless system and method for scanning a photoreceptor surface is described. In U.S. Pat. No. 6,008,653, entitled CONTACTLESS SYSTEM FOR DETECTING MICRODEFECTS ON ELECTROSTATOGRAPHIC MEMBERS, a contactless process is disclosed for detecting surface potential charge patterns in an electrophotographic imaging member, including applying a constant voltage charge to an imaging surface of a photoreceptor, and biasing a capacitive scanner probe having an outer shield electrode to within about ±300 volts of the average surface potential of the imaging surface. The probe is maintained adjacent to and spaced from the imaging surface to form a parallel plate capacitor with a gas between the probe and the imaging surface. Relative movement is established between the probe and the imaging surface, maintaining a substantially constant distance between the probe and the imaging surface. The probe is synchronously biased and variations in surface potential are measured with the probe. The surface potential variations are compensated for variations in distance between the probe and the imaging surface. The process described in U.S. Pat. No. 6,008,653 is implemented using a system for maintaining a substantially constant distance between the probe and the imaging surface. This system is described in U.S. Pat. No. 6,119,536, entitled CONSTANT DISTANCE SCANNER PROBE SYSTEM. While ideally the distance between the probe and the imaging surface is maintained constant while scanning the imaging surface, in reality small variations do occur. An algorithm is provided for compensating for variation in the distance between the probe and the imaging surface. The algorithm is based on compensation for a flat plate capacitor in which a known time-varying reference charge (or voltage) is applied to the drum or conductive substrate of the material-under-test and is uniformly spatially distributed below the probe surface. However, when studying and analyzing the latent image, the charge distribution on a surface can be non-uniform.
Another technique related to sensing electrostatic surface potentials can be found in U.S. Pat. No. 7,271,593, entitled CONTACTLESS SYSTEM AND METHOD FOR DETECTING DEFECTIVE POINTS ON A CHARGEABLE SURFACE, the entire disclosure thereof being incorporated herein by reference. The patent describes a device and method for detecting charge deficient spots (CDS's) in a latent image on a photoreceptor for use in xerography. The disclosed device makes use of a chargeable photoreceptor surface held at a first voltage, the surface of a probe held at a second voltage, determining the potential of a CDS on the chargeable surface based on received readings from the probe, and then applying a reference charge to the chargeable surface and thus correcting the CDS. Furthermore, the current device includes higher order mathematical corrections to account for the point-like nature of the CDS's.
While the methods discussed here are highly effective for detecting transient events such as a CDS, which appear as small, spatially localized discharged spots or defects on the uniformly charged photoreceptor surface, the apparatus or instrument described detects transient changes in the surface voltage, as caused by a CDS. The disclosed devices and methods of the prior art are not capable of sensing absolute surface voltage. The current invention detects the absolute surface voltage through the vibration of the probe, a feature which is strongly desired when detecting a latent image for xerography purposes or when studying latent images.
Thus, there is a need for a system and a method which directly measures charge migration or surface voltage at high resolution, is easy to use, and can be immediately implemented into photoreceptor research.